1. Field of the Invention
The present invention relates to thermosensitive recording materials comprising a support containing a substantially colorless or light-colored electron donative leuco dye and a color developer of an electron accepting substance color.
2. Discussion on Related Arts
Thermosensitive recording materials are generally composed of a support such as paper, synthetic paper, a plastic film, etc. having provided thereon a thermosensitive color forming layer containing mainly a composition capable of forming a color with heat, in which colored images are obtained by heating with a thermal head, a thermal pen, a laser light, a strobo flush, etc. This type of recording material has such advantages that recording can be made using a relatively simple equipment in a short period of time without such complicated processings as development, fixing, etc. compared to other recording materials, causing noise and environmental pollution is less, its cost is relatively low and the like. Thus, these materials have been widely used as recording materials in various areas including electronic computers, facsimile machines, ticket vending machines, labellers, recorders, etc., in addition to making copies of books, documents, etc.
Thermosensitive recording materials are generally prepared by separately grinding colorless or light-colored leuco dyes of lactone, lactam or spiropyrane type as a color former and a color developer capable of forming a color by heating the color former, for example, a phenolic substance, an organic or inorganic acidic substance or its ester or salt, etc. with a ball mill or an attritor and the like into an optional particle size, adding a binder to the ground particles, mixing them, if necessary and desired, further incorporating pigments and additives into the mixture and then applying the resulting mixture onto a support, for example, paper, by means of coating with an air knife coater, etc. and drying.
In order to improve color sensitivity further, smoothness of the surface of the thermosensitive color forming layer is enhanced via a machine calendering treatment or a supercalendering treatment, etc., after the application followed by drying described above, whereby close contact with a thermal head is achieved.
Furthermore, many proposals have been hitherto made to provide an intermediate layer between a support such as paper and a thermosensitive color forming layer, in order to attain super high sensitivity and at the same time, to minimize adherence of thermally fused matters onto the thermal head (hereafter referred to as foreign matters adhered to head).
As known substances for the intermediate layer or known intermediate layers heretofore proposed, there are, for example, an intermediate layer of synthetic paste, starch or gelatin described in Japanese Patent application KOKAI (Laid-Open) No. 48-10942; an intermediate layer containing calcium carbonate and inorganic powders showing oil absorption of 50 ml/100 g or more described in Japanese Patent application KOKAI (Laid-Open) No. 54-23545; an intermediate layer composed of finely divided particles of synthetic resin and a binder described in Japanese Patent application KOKAI (Laid-Open) No. 55-86789; an intermediate layer containing urea or thiourea derivatives described in Japanese Patent application KOKAI (Laid-Open) No. 59-184691; an intermediate layer composed of a filler, wax or a wax-like substance and a binder described in Japanese Patent application KOKAI (Laid-Open) No. 59-204594; an intermediate layer containing bisphenol derivatives described in Japanese Patent application KOKAI (Laid-Open) No. 60-122191; an intermediate layer using polystyrene and silicon oxide in combination described in Japanese Patent application KOKAI (Laid-Open) No. 61-89883; an intermediate layer containing either a 2,3-dihydroxynaphthalene compound or a metal compound described in Japanese Patent application KOKAI (Laid-Open) No. 61-164882; an intermediate layer containing the same color developer as used in the thermosensitive color forming layer described in Japanese Patent application KOKAI (Laid-Open) No. 61-274989; an intermediate layer containing a phenolic antioxidant described in Japanese Patent application KOKAI (Laid-Open) No. 62-39281; an intermediate layer containing 4,4'-bisphenolsulfone monoether compound described in Japanese Patent application KOKAI (Laid-Open) No. 62-149477; an intermediate layer containing a diphenolic antioxidant described in Japanese Patent application KOKAI (Laid-Open) No. 62-167076; an intermediate layer containing a benzotriazole compound described in Japanese Patent application KOKAI (Laid-Open) No. 62-176879; etc.
It is currently general to take so-called dual structure that these substances for the intermediate layer are coated onto a support in a coverage or coating amount of 1 to 15 g/m.sup.2 (dry solid weight) depending upon purpose of use and providing thereon a thermosensitive color forming layer.
By taking the dual layer structure as described above, improvement in sensitivity, reduction in foreign matters adhered to head or improvement in storability of colored images are achieved but the issues of sensitivity and foreign matters adhered to head are yet unsatisfactory. For purposes of further improving smoothness thereby to improve close contact with a thermal head, large pressure is applied by supercalendering, etc. so that a color forming reaction undesirably occurs between a leuco dye and a color developer in a thermosensitive color forming layer to cause a color stain on thermosensitive paper, that is, background fog occurs.
To improve these problems, so-called three layer structure providing the first and second intermediate layers on a support and then the thermosensitive layer thereon has been proposed. There have been proposed, for example, a method of providing the first intermediate layer containing a foamable plastic filler and the second intermediate layer containing a filler and a binder in Japanese Patent application KOKAI (Laid-Open) No. 59-225987; a method of providing a film-forming polymer as the first intermediate layer and a composition mainly composed of a pigment showing an apparent specific gravity of 0.5 g/cm.sup.2 or below as the second intermediate layer in Japanese Patent application KOKAI (Laid-Open) No. 61-11286; a method of providing an inorganic pigment having oil absorption of 100 ml/100 g or more as the first intermediate layer and a film-forming high molecular substance layer as the second intermediate layer in Japanese Patent application KOKAI (Laid-Open) No. 61-193880; a method of providing the first intermediate layer mainly composed of polyvinyl alcohol and the second intermediate layer mainly composed of finely divided hydrophobic high molecular particles in Japanese Patent application KOKAI (Laid-Open) No. 61-229589, etc.
According to replication by the present inventors, it was confirmed that some of these proposals achieved great improvement in sensitivity; however, it was observed that the foreign matter problem was rather accelerated.